Why were submarines significant in World War 1?

Submarines were significant in World War I because they
inflicted a lot of damage on shipping around Britain. They sank
both military ships, supply ships, and anything else, which brought
out the term unrestricted submarine warfare. To site a single
example, the U-9 sunk three British cruisers within an hour.
We also saw the strategy of the convoy appear in defense. The other
significance of submarine warfare in World War I is that the
capability of the submarine was clearly demonstrated, and the
strategy and tactics (as well as the technology) was developed even
further. World War II submarine activity was an extension of the
lessons provided in the first war.